I definitely think a rule should be broken and sometimes to get a great shot you cannot sit and try and compose a shot but we need to be aware of the rules.

If you don't know the rules of composition then you can't break them. Once we understand undeniable rules in art, then knowing that rule we should make a conscious decision to break it.

Again, sometimes when I come into a situation I will fire off a shot or two and then try and compose it. Most my photography is on location, candid, lifestyle photography. Sometimes, I do not have time to compose. But, I find that I have begun to -due to repetition- use the rule of thirds on reflex. It helps to try and always pay attention to other rules of composition too, not just rule of thirds.

In my opinion, it is better to rely on the rule of thirds as a standard when you are taking pictures. The rule of thirds is a very useful too to have to make your photography better.